The present invention relates to a fixing device which is used in an image forming apparatus according to the electrophotographic system such as a copying machine, a printer, or a facsimile apparatus to heat and fuse a developer image that has been transferred from an image bearing body to a transfer sheet such as normal paper or a transparency resin sheet for an overhead projector (OHP sheet), thereby fixing the image to the sheet.
Conventionally, as shown in FIG. 46, a known fixing device of this kind comprises fixing members 3 and 4 configured by a pair of rollers in each of which an elastic member 2 is disposed on the outer periphery of a metal cylinder 1, heating means 5 for heating the fixing members 3 and 4, driving means 6 for rotatingly driving at least one fixing member 3, and a spring 7 which causes the fixing members 3 and 4 to contact with each other at a constant abutting force. A transfer sheet 9 is transported to a contact portion (nip portion 8) between the elastic members 2 of the pair of fixing members 3 and 4. A developer on the transfer sheet 9 is fused by heat supplied from the fixing members 3 and 4 and then fixed to the transfer sheet 9.
In order to improve the productivity in a copying machine, a printer or the like, the rotational speeds of the fixing members 3 and 4 should be increased so as to increase a speed at which the transfer sheet 9 is passed through the nip portion 8 (fixing speed).
In the conventional fixing device, silicone rubber constituting the elastic members 2 has a heat conductivity of about 0.6 to 0.8.times.10.sup.-3 cal/cm.multidot.sec.multidot..degree. C. Therefore, the efficiency of absorbing heat from the surfaces of the elastic members 2 into the transfer sheet 9 passed through the nip portion 8 is higher than that of supplying heat from the heating means 5 to the surfaces of the elastic members 2. As a result, a phenomenon takes place in which, when the fixing speed is set to be higher, the surface temperatures of the elastic members 2 are temporarily decreased during a period when an operation of continuously supplying sheets is started. The decrease of surface temperature causes a failure in image gloss or a fixing failure.
In order to increase the efficiency of the heat supply from the heating means 5 to the surfaces of the elastic members 2, a countermeasure may be taken in which a reinforcing inorganic filler such as silica or iron oxide added to the silicone rubber is replaced with a high thermal conductive filler such as alumina or magnesium oxide, and the addition amount of the filler is increased. However, a high thermal conductive filler has no effect of improving the heat resistance. Furthermore, as the addition amount of the filler is increased, the polymer component having excellent heat resistance is inevitably reduced. Therefore, the elastic members are decreased in strength in high temperature and hence susceptible to be thermally degraded. In the case where the contact abutting force between the fixing members 3 and 4 is constant as in the fixing device shown in FIG. 46, when the hardness is reduced as a result of thermal degradation, the width of the nip portion 8 (nip width) is increased. When the nip width is increased, the quantity of heat supplied from the nip portion 8 to the transfer sheet 9 is varied and hence the image quality is changed.
Further, when the addition amount of the filler is increased, the hardness of the silicone rubber is increased and the elastic members 2 are decreased in elasticity. When the elasticity of the elastic members 2 is decreased, it is necessary to perform the abutting of the fixing members 3 and 4 at a very high pressure or to thicken the elastic members 2 in order to ensure the width of the nip portion 8, thereby causing an image to be obscured or a sheet to become crinkled, or reducing the heat supply quantity to cause a fixing failure or the like.
Furthermore, when the density of cross-link of the silicone rubber is increased higher by changing the polymer component, it is possible to suppress the reduction of the hardness due to the thermal degradation. However, the increased density of cross-link causes the elasticity of the elastic members 2 to be decreased.